1. Field of the Invention
This invention relates to analog multiplier circuits, and more particularly to an analog multiplier circuit which operates at low power supply voltages.
2. Description of Related Art
Multipliers are key components in communications equipment, such as radios. The frequency of the output of the mixer is the product of the frequency of a local oscillator (LO) signal and an intermediate frequency (IF) signal. Multipliers which are not linear with respect to both inputs are commonly referred to as mixers. Mixers are used to down-convert (i.e., translate to a lower frequency) a signal received by a radio front end. This process, called heterodyning, allows signals to be broadcast at relatively high frequencies, and then down-converted to intermediate frequencies which are easier to process within a radio. Likewise, when broadcasting, a radio will modulate information upon an intermediate frequency signal. This intermediate frequency signal is then up-converted to an RF signal which is then broadcast.
FIG. 1 is a circuit diagram of a prior art mixer circuit commonly referred to as a "diode-ring" mixer. This mixer is shown for down-conversion. That is, an RF signal and an LO signal are input to the mixer to generate an IF output signal which has a frequency that is equal to the frequency of the RF signal minus the frequency of the LO signal. This circuit requires an LO signal with a relatively high power level. The diode-ring circuit shown in Figure 1 performs mixing on essentially voltage mode signals (diodes have a low on resistance, controlled by the LO signal). Typically, the LO drive must exceed the largest RF signal by a margin sufficient to ensure that the RF signal does not switch the diodes of the mixer when the polarity of the RF signal opposes the polarity of the LO signal. For example, a "Level 17" mixer requires +17 dBm of LO drive to handle an RF signal of +10 dBm. Also, to reduce non-linearity, the diodes are sometimes operated at extremely high currents.
Furthermore, because of the highly non-linear nature of diodes, the impedance of each of the three ports is also non-linear. That is, the impedance at each port will vary with changes in the signal level. Consequently, exact impedance matching at the ports is not possible. Furthermore, there is substantial coupling between the three ports which, taken together with the high LO power needed, makes it likely that a distorted form of the LO signal will be present in the RF output. Additionally, the circuit shown in Figure 1 is not capable of providing conversion gain. In fact, the minimum theoretical loss is (3.9 dBm). Practical mixers using the circuit of FIG. 1 have losses which are always higher than the theoretical minimum.
In an attempt to design a mixer that overcomes some of the disadvantages of the circuit shown in FIG. 1, the circuit shown in FIG. 2 was developed. The circuit shown in FIG. 2 is often referred to as a "Gilbert" mixer. Gilbert mixers can be fabricated .in monolithic form. Furthermore, Gilbert mixers can provide conversion gain and require relatively low power at the LO port. Still further, Gilbert mixers provide high isolation between ports and are relatively insensitive to the impedance of devices coupled to the input ports.
Active mixers (such as the Gilbert mixer of FIG. 2) operate in current-mode. Transistors Q3-Q6 are controlled by current from the voltage-current converter Q 1, Q2. The output is provided across load resisters RL1 and RL2. The use of a differential output allows the voltage gain to be doubled.
The Gilbert mixer has good port-to-port isolation, because the Gilbert mixer is "doubly-balanced". That is, in the absence of any RF signal (i.e., a voltage difference between Q1 and Q2) the collector currents are essentially equal, and the voltage at the LO results in no change of the output signal. In addition, if an RF signal is applied, but no voltage difference is present at the LO input, the outputs are likewise balanced. Only when a signal is present at both ports (LO and RF) is an output signal generated. In addition, the Gilbert mixer of FIG. 2 provides better isolation than the diode ring of FIG. 1 due to the reversed biased transistor junctions of the Gilbert mixer.
However, the supply voltage (V+) used with Gilbert mixer must be high enough to allow linear operation of each of the six transistors Q1-Q6. If any of the transistors Q1-Q6 begin operating outside the linear operating region, the IF output will become distorted. Therefore, the supply voltage used to power a Gilbert mixer must typically have a voltage of at least approximately 2.0 volts to 2.5 volts to ensure that the transistors Q1-Q6 remain in the linear region of operation. However, if an active constant current source is provided in the tail of each differential pair, then even greater voltage is required.
Because power supply demands are preferably minimized in portable communication systems, it would be desirable to provide a mixer which operates linearly at relatively low supply voltage levels, has robust output levels, provides gain conversion, and can be manufactured using conventional monolithic or integrated circuit techniques. It can be seen from the above description of the Gilbert mixer that the LO signal, the RF signal, or the IF signal may be applied to either of the two inputs. In the case in which down-conversion is desired, the RF and LO signals are each applied to one of the two inputs such that the output is an IF signal. In the case in which up-conversion is desired, the IF and LO signals are each applied to the one of the two inputs such that the output is an RF signal. The present invention provides such a circuit.